Dust stop systems for internal mixers--part 2.

Today, the production of rubber compounds consisting of a number of ingredients mainly takes place in internal mixers. The rubber compounds include polymers, finely dispersed powder fillers, carbon black, chalks and smaller quantities of chemicals, curatives, accelerators or processing aids, and liquids such as plasticizer oils. The emission of these liquids and finely dispersed ingredients from the mixer into the surrounding atmosphere is restricted and controlled by the use of dust stops.

At first, the finely dispersed ingredients are pasted with an oil film, called paste-bonding. Due to the high pressure inside the mixing chamber, the paste created is slowly squeezed outwards along the rotor shafts. To restrict the flow of the paste, a pair of rings seals the rotor shafts at each end.

The seal itself consists of two superimposed rings which are generally equipped with hard-coated surfaces in the sliding area. One ring is fixed on the rotor shaft; the counter-ring is mounted to the end frame (ref. 1).

The dust stop systems essentially differ in the way the surface pressure is applied. One differentiates self-sealing dust seals (SSA); spring-loaded dust seals (GA): and hydraulic dust seals (WYH).

Last month's Process Machinery column (Rubber World, November 2005) described dust stop design, the lubrication system and problems of dust stops, and analyzed principle procedures in the annular gap of dust stop seals. An analysis of process parameters is provided below, and lubricant-free dust stop systems are described.

Analysis of process parameters

A relevant factor concerning the wear of the sealing rings and the development of the disposal is the application of an adjusted contact pressure. As an important condition to assure a reliable sealing function, the sealing rings need to be prevented from opening. This means that the surface pressure of the sealing rings must exceed the pressure build-up in front of the dust stops.

On the one hand, a reliable sealing needs to be ensured; on the other hand, the surface pressure should be reduced to a minimum being necessary to minimize the wear of the rings. For this purpose, it is necessary to know the pressure build-up in front of the sealing rings.

To acquire these data, the fixed sliding ring of a GA-dust stop has been equipped with a pressure sensor. This sensor shall provide information on the pressure development in the mixing chamber during the mixing process.

Process parameters of an EPDM recipe, mixed in the conventional way, are described. First, the polymer, together with the carbon black, is added, followed by a mixing step of 45 s. At this moment, the fill factor accounts for 47%. The first power peak is reached after the drop of the ram. In the case of the under-filled mixing chamber, no or only a slight pressure build-up can be recognized. The second ram lift is followed by the addition of other fillers and process oil. The fill factor is now 87%. The following drop of the ram results in a sudden pressure increase. Here, a cyclic sequence of pressure maxima and minima can be observed.

A detailed examination of the maxima and minima is observed in the phase of the second ram setting where the pressure build-up consists of two parts. One part results from the pressure increase in the mixing chamber; the other part results from the movement of the rotor shaft during the rotation. Consequently, one can differentiate an axial pressure build-up and a radial pressure build-up.

The axial pressure build-up firstly results from the pressure increase caused by the ram setting. In addition, the material transport along the rotor shafts due to the Weissenberg effect increases the load of the dust stops. In order to prevent the dust stops from opening, the surface pressure has to exceed the axial pressure build-up.

Lubricant-free dust stop systems

A significant reduction of the required amounts of process oil, as well as the corresponding quantities of oil disposal, can be obtained by the use of lubricant-free dust stops. This way, lubricant-free dust stops provide the opportunity to reduce costs and also to improve the compound quality as just described (ref. 6). Furthermore, a special advantage is the possibility to refit internal mixers which are already used in production.

The authors Zaczek and Reardon (ref. 6) suggest the use of thermoset materials like cured carbon fiber reinforced polyimide, and engineering thermoplastics like fiber reinforced polyetheretherketone (PEEK). The problem in the application of soft polymer materials is given by the embedding of abrasive particles in the polymer matrix which can cause accelerated wear of the hard counterpart. The wear behavior of both materials can be improved by increasing the hardness of one partner (ref. 4). One approach is to strengthen the polymer matrix by fiber reinforcement. The other approach is the application of special wear resistant coatings in order to enhance the resistance of the hard counterpart material. The investigations of Jacobs et al. (ref. 2) and Lu et al. (ref. 3) confirm the positive impact of hard counterparts.

Therefore, practical field tests were carried out on a GK 135 E, varying both the polymer material and the wear resistant coating of the steel ring. The conditions of use can be summarized as follows:

* GA dust stop;

* no lubrication of the sliding rings;

* retention of the paste bonding; and

* all kinds of technical rubber goods compounds, from masterbatches to final batches (carbon black compounds).

Regular checks guaranteed the documentation of the wear behavior. For this purpose, the rotating polymer rings were removed from the internal mixer. After cleaning the rings, the value of wear was determined.

Wear development of the different material combinations as a function of the operating period is described. Material combination 1 (coating A/thermoplastic 1) exhibits an excellent wear rate. The sealing rings were removed from the mixer after an operating period of 37 months. By extrapolating the wear up to the possible maximum wear distance, it can be seen that a further operating time of ten months would have been possible. A similar wear behavior can be determined regarding material combination 2 (coating b/thermoplastic 2). However, these rings had to be removed after an operating period of 16 months because of too little wear resistance and too little thickness of the coating at the same time. The test duration of the presently installed dust seals three to five is not completed yet. Hence, no concluding judgement can be made at this point in time.

The effect of lubricant-free dust stops is also analyzed concerning the reduction of oil disposal. The development of the amount of oil disposal was compared to the conventional lubricated dust stop system over a period of six months. The oil, used for the paste bonding in the case of the lubricant-free dust stops, was collected separately for each dust stop over a period of four weeks. Afterwards, the oil quantities were standardized on the hours of operating and related to the conventional lubricated system. It was proven that it is possible to reduce the average oil consumption by 72%. Nevertheless, the retention of the paste bonding is necessarily recommended, even in the case of the use of lubricant-free dust stops.

Conclusions

An adjusted setting of the contact force of the dust stops is a significant factor concerning the wear sealing rings, in phases of strong dust stop loads, it is very important to keep the sealing closed. Material which penetrates the sliding area may lead to accelerated wear. It takes a long time to close an already opened dust stop. If the surface pressure is increased too late, the abrasive particles are pressed into the sliding surfaces, and thereby they increase the wear rate of the rings. Consequently, the opening of the sealing must be prevented in advance. The measurement of the pressure build-up in front of the dust stops provides the opportunity to detect these critical process phases. On this basis, the required contact force can be judged.

By the use of lubricant-free sealing systems, a significant reduction of process oil can be obtained. At the same time, the oil disposal could be reduced by 72% compared to the lubricated system. As a result, costs for oil purchase and oil disposal are lowered, while product quality can be improved. The retention of the paste bonding is recommended, even with the use of lubricant-free dust stops, in order to paste the finely dispersed ingredients and to rinse the annular gap so that no material can get stuck and start to vulcanize. Furthermore, the material transport caused by the Weissenberg effect can be reduced.

The tested material combinations show an excellent wear behavior. Service lives of at least 37 months are realized. Further improvements could be obtained by the application of very hard and smooth counterparts for the polymer materials. Together with an adapted adjustment of the surface pressure, obtained by pressure measurements in front of the dust stops, the service life of the sealing rings can be extended.

References

(1.) Berkemeier, D., "Internal mixer--mechanical engineering aspects, internal workshop." Mixing rubber compounds," VDI-Gesellschaft Kunststofftechnik, 2002.

(2.) Jacobs, O., Friedrich, K and Schulte, K., "Fretting wear of continuous fiber-reinforced polymer composites," ASTM Special Technical Publication, Vol. 1,167, 81, 1993.

(3.) Lu, Z., Friedrich, K., Pannhorst, W. and Heinz, J., "Wear and friction of a unidirectional carbon fiberglass matrix composite against various counterparts," Wear, 162-164 (1993), 1,103-1,113.

(4.) Uetz, H., Abrasion and Erosion, Carl Hanser Verlag, 1986.

(5.) Woydt, M., "Tribologische werkzeugkonzepte fur den trockenlauf " Tribologie Keramischer Werkstoffe (605), 2001.

(6.) Zaczek, L. and Reardon, J., "Advantages of oil-less dust stops for mixers, "Rubber World, April, 2002.